We have reported on the presence and the predominant forms of pro-opiomelanocortin (POMC)-derived peptides in rat testicular homogenates and on immunostainable Beta-endorphin and N-terminal ACTH in Leydig cells and epithelia of other portions of the male reproductive tract. We now propose a study to determine: (1) if rat Leydig cells in culture synthesize POMC-like material and POMC-derived peptides; (2) if rat Leydig cells in culture secrete POMC-derived peptides; (3) if POMC-derived peptides can be detected in spermatic vein of hypophysectomized animals; and (4) factors involved in the regulation of synthesis and secretion of POMC and POMC-derived peptides in vitro and in vivo. To accomplish this we plan to study: (A) the incorporation of 3H-lysine into POMC-like material by Leydig cells in culture; (B) the secretion of tritiated POMC-derived peptides into the media by Leydig cells in culture after incubation with 3H-lysine; (C) the effect of substances known to affect Leydig cell function and pituitary POMC on both synthesis and secretion rates of this tritiades POMC-like and POMC-derived material; and (D) the in vivo (spermatic vein) secretion of POMC-derived peptides, as determined by radioimmunoassay, in hypophysectomized animals, and their reponse to systemic CRF and hCG administration. It appears that POMC-derived peptides may have a paracrine role in the male reproductive system. Elucidation of their synthesis, regulation, and their secreted forms will enable further studies to ascertain their physiological role. The delineation of their hitherto unsuspected role in the male reproductive system should also lead to new approaches in the elucidation of disease of this system.